Traditionally, well testing is conducted using a mono-bore conduit to convey production fluids between the marine wellhead, at the mud line, and the xmas tree/flow head at the surface facilities. Various regulatory bodies require the establishment of two barriers between the reservoir and the environment. A mono-bore sub-sea test tree is used to facilitate pressure control. This test tree contains two separate valves to provide the mandatory barriers in the production fluid flow path in order to enable the well to be closed in. The primary annulus barrier is the production packer and the secondary annulus barrier is provided by the contact of blow-out preventer (BOP) pipe rams, seals with the mono-bore riser. Access into the annulus, between the production tubing and the production casing, is required to enable the annulus pressure to be monitored and to be adjusted as necessary. In the traditional systems the annulus flow path is vertical up to the isolation point at which the blow-out preventer (BOP) stack pipe ram seals contact the mono-bore riser. The vertical passage of annulus fluid is blocked at the aforementioned seal and the fluid passage to surface is via a hydraulically actuated valve of the BOP system into external choke or kill lines which are attached to the BOP stack and the marine riser.
U.S. Pat. No. 4,784,225 discloses a well valve assembly for the control of well fluids that flow in the tubing and annulus.